Structural member of automobile and automobile body comprising it

ABSTRACT

The present invention provides a structural member for a car capable of demonstrating excellent flexural strength and absorbed energy performance and, moreover, capable of realizing weight reduction and is a structural member for a car being formed with a high-tensile steel pipe having a material property of not less than 1,400 MPa in tensile strength and not less than 5% in elongation, and also having a closed section structure as the configuration of the cross section perpendicular to the direction of the longitudinal axis, the cross-sectional configuration satisfying the expression (1) below when defining a as the maximum length of said cross-sectional configuration in the longitudinal direction, when defining b as the maximum length thereof in the direction perpendicular to said direction, when defining L as the circumferential length thereof and when defining t as the wall thickness of the steel pipe, 
     0.65≦ b/a ≦0.75, 
     0.014≦ t/L ≦0.020  (1); 
     and preferably, being disposed so that the longitudinal direction of the cross-sectional configuration of the structural member coincides with the direction of the collision of a car body when the structural member is attached to the car body.

TECHNICAL FIELD

[0001] The present invention relates to a structural member for a car, such as a door-impact-protection beam which is a door stiffening member, a pillar stiffening member, a bumper stiffening member, etc., capable of demonstrating excellent flexural strength and absorbed energy performance, and moreover, capable of realizing weight reduction and to a car body fabricated using the structural member for a car.

BACKGROUND ART

[0002] Up to now, a structural member for a car, such as a door-impact-protection beam which is a door stiffening member, has been widely used for securing the safety of a driver in the case of a lateral collision of a car. As a door-impact-protection beam, an ultra-high-tensile steel pipe has been adopted for securing collision safety and realizing low fuel consumption by weight reduction. As a material for such a steel pipe, a material having a tensile strength of not less than 1,400 MPa and a cross section of a perfect circle has been generally used.

[0003] It is now required to further reduce fuel consumption by reducing the weight of car components and the like, and thus a lighter door-impact-protection beam in weight is required. Therefore, a door-impact-protection beam having the thinnest possible wall thickness is desired. However, as a door-impact-protection beam is used for securing safety at the time of collision and is required to secure flexural strength and absorbed energy performance more than a certain level, the weight reduction of a conventional door-impact-protection beam having a cross section of a perfect circle has reached a limit.

[0004] The object of the present invention is to provide a structural member for a car, such as a door-impact-protection beam which is a door stiffening member, a pillar stiffening member, a bumper stiffening member, etc., which structural member being capable of solving the above-mentioned conventional problems, having flexural strength and absorbed energy performance equal to or exceeding a conventional steel pipe with a circular cross section when a car collision (for example, a head-on collision, a lateral collision, etc.), particularly a lateral collision, occurs, and moreover being capable of attain weight reduction further than a conventional structural member and a car body fabricated using said structural member for a car.

DISCLOSURE OF THE INVENTION

[0005] The present invention was established for solving the above-mentioned problems and the gist of the present invention is as follows:

[0006] (1) A structural member for a car, characterized by being formed of a high-tensile steel pipe having a material property of not less than 1,400 MPa in tensile strength and not less than 5% in elongation, and also having a cross-sectional configuration of a closed section structure and the cross-sectional configuration satisfying the expression <1> when defining a as the maximum length of said cross-sectional configuration in the longitudinal direction, when defining b as the maximum length thereof in the direction perpendicular to said direction, when defining L as the circumferential length thereof and when defining t as the wall thickness of the steel pipe,

0.65≦b/a≦0.75,

0.014≦t/L≦0.020   <1>.

[0007] (2) A structural member for a car according to the item (1), characterized in that said cross-sectional configuration has a linear portion at at least one portion of the sides.

[0008] (3) A structural member for a car according to the item (1), characterized in that all of the sides of said cross-sectional configuration are formed with curves.

[0009] (4) A structural member for a car according to the item (1), characterized in that said cross-sectional configuration is an ellipse.

[0010] (5) A structural member for a car according to any one of the items (1) to (4), characterized in that said member is disposed so that the longitudinal direction of the cross-sectional configuration thereof coincides with the direction of the collision of a car body when the structural member is attached to the car body.

[0011] (6) A structural member for a car according to any one of the items (1) to (5), characterized in that said member is used as a door-impact-protection beam disposed so that the longitudinal direction of the cross-sectional configuration thereof coincides with the direction of the lateral collision of a door when the structural member is attached to the door.

[0012] (7) A car body characterized in that a structural member for a car according to any one of the items (1) to (6) is disposed so that the longitudinal direction of the cross-sectional configuration of said structural member coincides with the direction of the collision of the car body.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a graph showing the relation of b/a with the maximum load and the absorbed energy when quadrilateral beams, which have an identical weight to a beam having a circular cross-section of a diameter of 31.8 mm×a thickness of 1.6 mm, and the ratios of b to a being changed variously, are subjected to the three-point bending test at the span of 950 mm.

[0014]FIG. 2 is a graph showing an example of the relation between the wall thickness t and the circumferential length L.

[0015]FIG. 3 is a graph showing the relation between the displacement and the load when the test pieces are subjected to the three-point bending test at the span of 950 mm.

[0016]FIG. 4 is a sectional view showing an embodiment of the present invention.

[0017]FIG. 5 is a sectional view showing another embodiment of the present invention.

[0018]FIG. 6 is a sectional view showing yet another embodiment of the present invention.

[0019]FIG. 7 is a sectional view showing yet another embodiment of the present invention.

[0020]FIG. 8 is a sectional view showing yet another embodiment of the present invention.

[0021]FIG. 9 is a graph showing the results of the three-point bending test in the invented examples and the comparative examples.

[0022]FIG. 10 is a graph showing the absorbed energy in the three-point bending test.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] The preferable embodiments will be explained hereunder by taking, as an example, the case where a structural member for a car according to the present invention is used as a door-impact-protection beam attached to a car body and by referring to the drawings.

[0024] A door-impact-protection beam according to the present invention is formed with a high-tensile steel pipe whose cross-section perpendicular to the long axis has a closed section structure, which steel pipe is formed of a steel strip having a material property of not less than 1,400 MPa in tensile strength and not less than 5% in elongation and a chemical composition containing, in weight, for example, C: 0.24%, Si: 0.25%, Mn: 2.4%, Cr: 0.5%, Mo: 0.7%, Ti: 0.03%, B: 20 ppm, the balance consisting of Fe and unavoidable impurities.

[0025] The cross-sectional configuration satisfies the expression <1> when defining a as the maximum length of said cross-sectional configuration in the longitudinal direction, when defining b as the maximum length thereof in the direction perpendicular to said direction, when defining L as the circumferential length thereof and when defining t as the wall thickness of the steel pipe,

0.65≦b/a≦0.75,

0.014≦t/L≦0.020  <1>.

[0026] Preferably, the beam is disposed so that the longitudinal direction of the cross-sectional configuration thereof coincides with the direction of the lateral collision when the beam is attached to a door.

[0027] The cross-sectional configuration of a beam according to the present invention is designed based on the configuration which is inscribed in a rectangle, each corner portion of which is rounded in the shape of a circular arc. By forming each corner portion in a shape of a circular arc, the occurrence of cracks and buckling, caused by excessive stress concentration at each corner portion when an impact load is imposed, is prevented.

[0028] The reasons for limiting the above-mentioned dimensions of the cross-sectional configuration will be explained hereunder.

[0029] (1) Firstly, the limiting conditions of b/a will be explained.

[0030]FIG. 1 is a graph evaluating the bending property in terms of the maximum load and the absorbed energy amount when the steel pipes having an identical cross-sectional area are subjected to the three-point bending test with a load being imposed from the same direction as the longitudinal direction of the cross-sectional configuration, while varying the values of b/a.

[0031] When the value of b/a is gradually reduced from a round shape (b/a=1), both the maximum load and the absorbed energy increase, and they radically increase in the range of 0.65≦b/a≦0.75. In the range of b/a<0.65, though the bending property still improves, the long side a (the maximum length in the longitudinal direction) becomes too large compared with the short side b (the maximum length in the direction perpendicular to the longitudinal direction), thus the beams in their configurations become unstable, the twist and rotation of the beams occur at the time of a lateral collision, and therefore, the original performance of the beams may not be demonstrated and a problem of safety may occur. Another possible problem is that the beam may not be accommodated in a door because of the constraint on the space.

[0032] For that reason, it is preferable that b/a is controlled in the range of 0.65≦b/a≦0.75.

[0033] (2) Secondly, the limiting conditions of t/L will be explained.

[0034] For determining a cross-sectional configuration effective for securing the safety at the time of a collision, in addition to the value of b/a, the ratio of the wall thickness t to the circumferential length L, namely t/L, is an important parameter.

[0035]FIG. 2 is a graph showing the relation between t and L when b/a is in the above range (0.65≦b/a≦0.75).

[0036] In the range of t/L<0.014, since L is large compared with t, buckling tends to occur and cracking, after the buckling, tends to occur. On the other hand, in the range of t/L>0.020, though the bending property is good, the weight tends to increase since t is large compared with L and that is contrary to the purpose of the invention.

[0037] When a beam is attached to an actual door, in addition to the limitation range of 0.014≦t/L≦0.020, L is restricted by the space in a door for attaching the beam and t is also restricted by the requirement of weight reduction. Moreover, L and t have to be in the region where the beam has sufficient bending property, and the optimum region (the portion of oblique lines in FIG. 2, in the case of K=140) is the region where the product of L and t satisfies the expression L×t>K (here, K is determined by the requirement for the performance of a door).

[0038] The more the mass distribution is apart from the neutral axis of the beam cross-section perpendicular to the longitudinal direction of the beam, the more the flexural rigidity in the direction perpendicular to the direction of the long axis of the beam (steel pipe) improves. Therefore, it is preferable that the cross-sectional configuration of a beam is, for example, an oval having linear portions along the two long sides as shown in FIG. 4 or a substantially rectangular shape having linear portions along the two long sides and two short sides perpendicular to the long sides as shown in FIGS. 5 and 6, because flexural rigidity in the direction perpendicular to the direction of the long axis of the beam (steel pipe) is greatly improved.

[0039] When the long sides have linear portions, there is a tendency to improve flexural rigidity in the direction perpendicular to the direction of the long axis of the beam (steel pipe) but to increase cracking after the occurrence of buckling. For alleviating the cracking caused by the buckling, it is preferable that the cross-sectional configuration has outward-convex curves along the long sides as shown in FIG. 7.

[0040] In the case of a beam having an oval cross-sectional configuration wherein the long sides and the short sides connect each other via smooth curves as shown in FIG. 8, though there is a tendency to make the retention of the oval configuration unstable and to generate twists and bends of the beam at the time of a collision, buckling hardly occurs and the cracking caused by the buckling becomes extremely small and, therefore, an oval cross-sectional configuration can be applied to a beam.

[0041] (3) Thirdly, the disposition of a beam in a door or a car body will be explained.

[0042] Since the cross-sectional configuration of a beam according to the present invention is not omnidirectional (not isotropic), which is different to a circular cross-sectional configuration, it is preferable to make the cross-sectional configuration directional so that the beam can demonstrate the maximum bending property when the beam is disposed in a door. That is, it is preferable to dispose a beam so that the longitudinal direction of the cross-sectional configuration of the beam coincides with the direction of the lateral collision when the beam is attached to a door. When a member is attached to a car body, it is preferable to dispose the member so that the longitudinal direction of the cross-sectional configuration of the member coincides with the direction of the collision. This is because the member is superior in flexural rigidity in the same direction as the longitudinal direction of the cross-sectional configuration as described above.

[0043] A structural member for a car according to the present invention can also be used as a pillar stiffening member, a bumper stiffening member or a structural member for a car body, in addition to a door-impact-protection beam.

EXAMPLE

[0044] Beams having cross-sectional configurations with the dimensions and the various ratios shown in the Examples 1 to 3 of Table 1 were subjected to the three-point bending test (speed: 2 mm/s, indentation: 200 mm) at the span of 950 mm (I=950 mm). The results were satisfactory in the evaluation of both the maximum load and the absorbed energy. On the other hand, as comparative examples, a conventional type of beam having a circular cross section 31.8 mm in diameter and 1.8 mm in wall thickness (Comparative Example 1) and a light-weight-oriented beam having a circular cross section 31.8 mm in diameter and 1.6 mm in wall thickness (Comparative Example 2) were subjected to the three-point bending test likewise. Here, Examples 1, 2 and 3 correspond to FIGS. 4, 5 and 6, respectively. FIG. 3 shows the load-displacement curves of Example 1 and Comparative Examples 1 and 2 in the three-point bending test. It should be noted that the absorbed energy is in the region indicated with the oblique lines at the lower portion of the above-mentioned load-displacement curve in the three-point bending test as shown in FIG. 10.

[0045] The results of the maximum load and absorbed energy obtained by subjecting the Examples and Comparative Examples to the three-point bending test are shown in FIG. 9.

[0046] As is apparent from the results, it was confirmed that the beams according to the present invention, while having the same weight as the light-weight type beam having the circular cross section 31.8 mm in diameter and 1.6 mm in wall thickness, showed the same performance in both the maximum load and absorbed energy as the beam having the circular cross section 31.8 mm in diameter and 1.8 mm in wall thickness, and demonstrated excellent configurational performances (FIG. 9). TABLE 1 Maximum Absorbed Tensile Elonga- load energy strength tion Dimension (kN) (J) (Mpa) (%) a b L t b/a t/L (1 = 950 mm) (1 = 950 mm) Example 1 1608 12.2 34 24.5 100 1.6 0.72 0.016 10.59 1427 Example 2 1625 11.8 33 23 100 1.6 0.70 0.016 11.18 1484 Example 3 1636 11.2 34 22 100 1.6 0.65 0.016 11.43 1539 Comparative 1642 12.2 31.8 31.8 100 1.6 1.00 0.016 9.07 1058 example 1 Comparative 1651 12.7 31.8 31.8 100 1.8 1.00 0.016 10.69 1422 example 2

Industrial Applicability

[0047] As is apparent from the above explanation, the present invention makes it possible to reduce the weight of a structural member for a car (for example, a door-impact-protection beam which is a door stiffening member, a pillar stiffening member, a bumper stiffening member) and a car body by demonstrating high flexural strength and absorbed energy performance compared with a conventional steel pipe with a circular cross section when a car collision, particularly a lateral collision, occurs. 

1. A structural member for a car, characterized by being formed with a high-tensile steel pipe having a material property of not less than 1,400 MPa in tensile strength and not less than 5% in elongation, and also having a cross-sectional configuration of a closed section structure and having a cross-sectional configuration satisfying the expression (1) below when defining a as the maximum length of said cross-sectional configuration in the longitudinal direction, when defining b as the maximum length thereof in the direction perpendicular to said direction, when defining L as the circumferential length thereof and when defining t as the wall thickness of the steel pipe, 0.65≦b/a≦0.75,0.014≦t/L≦0.020   (1).
 2. A structural member for a car according to claim 1, characterized in that said cross-sectional configuration has a linear portion at at least one portion of the sides.
 3. A structural member for a car according to claim 1, characterized in that all sides of said cross-sectional configuration are formed with curves.
 4. A structural member for a car according to claim 1, characterized in that said cross-sectional configuration is an ellipse.
 5. A structural member for a car according to any one of claims 1 to 4, characterized in that said member is disposed so that the longitudinal direction of the cross-sectional configuration thereof coincides with the direction of the collision of a car body when the structural member is attached to the car body.
 6. A structural member for a car according to any one of claims 1 to 4, characterized in that said member is used as a door-impact-protection beam disposed so that the longitudinal direction of the cross-sectional configuration thereof coincides with the direction of the lateral collision of a door when the structural member is attached to the door.
 7. A car body characterized in that a structural member for a car according to any one of claims 1 to 6 is disposed so that the longitudinal direction of the cross-sectional configuration of said structural member coincides with the direction of the collision of the car body. 